Pump



F. E. NORLIN March 29, 1960 PUMP 2 Sheets-Sheet 1 Q Q Q &

Filed July 18, 1956 Ina/677.2277 ffancwlfj orlzln Mud! 1950 F. E. NORLIN 2,930,321

PUMP

Filed July 18, 1956 2 Sheets-Sheet 2 States PUMP This invention relates to pumps in which a plurality of pump cylinders are arranged in circular series parallel to each other and of the axis of the drive shaft, each of the cylinders having disposed therein a piston or plunger actuated by a cam or swash plate fixed on the shaft.

The pump forming the subject matter of my invention is provided with means to vary the displacement thereof, such means including means responsive to the discharge pressure of the fluid from the cylinders, and the pump is further provided with means allowing a selection of a particular maximum discharge pressure therefrom. Such means are desirable in pumps which are constantly running, as for example, those used in aircraft installations and where the displacement requirements vary from a maximum to a zero value. The structure forming the subject matter of the instant application includes a variance of the means for accomplishing the desired results described and claimed in my co-pending application,

erial No. 573,543, filed March 23, 1956, and entitled Variable Displacement Hydraulic Pump.

Therefore, the primary object of my invention is the provision of an improved variable displacement pump of the type incorporating a plurality of circularly arranged cylinders having pistons therein actuated by means of a cam or swash plate. i

More particularly, my pump includes pistons, each having an open-ended cavity therein in which is disposed a valve means adapted to seat and close'the open end of the piston cavity, together with means for varying the position of closing the open end of the piston cavity by means of the valve during the piston discharge stroke to thereby effectively vary the effective discharge stroke of the piston and the displacement of the pump, the latter means including a pilot valve responsive to pump discharge pressure. In the aforementioned c'o-pending application, the means for varying the effective discharge stroke of the pistons includes a spider or a plate provided with a plurality of radially extending fingers movable by means of a spider piston responsive to a pressure related to the pump discharge pressure, so as to move the spider piston and, therefore, the spider between limits at which the pump will provide full and zero displacement, respectively.

The means for varying the effective discharge stroke of the pistons, forming the subject matter of the instant invention includes means comprising a plurality of sep-.

arate, individual, radially extending fingers, one for each piston, movable by means of a pressure related to the pump discharge pressure applied to the supporting stem of each finger, so as to move the stems and, therefore, the fingers between limits at which the pump will provide full and zero displacement, respectively. Therefore, an other object of my invention is the provision of an improved pump including a plurality of parallel cylinders, each having a piston reciprocated therein by means of a cam or swash plate, so as each to have a suction stroke and a discharge stroke, valve meansassociated with each of the pistons and adjustable so as to vary the discharge ice stroke of the piston to vary the displacement of the pump, and means individual to each valve means to provide to the adjustment thereof. a

More particularly, a still further object of my invention is the provision of an improved variable displacement piston swash type pump including means comprising a plurality of fingers, separate and individual to the pump pistons, and responsive to a fluid pressure related to the discharge pressure of the pump to vary the displacement thereof.

These and other objects and features of my invention will be apparent from the following specification when taken with the accompanying drawings, in which:

Figure 1 is a partial sectional view of a pump constructed in accordance with my invention;

Figures 2 and 3 are enlarged sectional views of pop tions of the pump illustrated in Figure 1; and

Figures 4, 5 and 6 are, respectively, side, rear and front views of one of the fingers. I

In each of the Figures 1 to 3 of the drawings the pump mechanism is illustrated in its full displacement position.

Statements of direction used in the following description refer to the pump positioned as in Figures 1 and 2 and are merely to facilitate description.

Referring now to the drawings, there is illustrated a pump, generally indicated at 10, having a housing comprising a body 11 and a cover 12. The cover 12 has an inlet connection 13 and an outlet connection'14 which take the form of tapped ports into which the suction and discharge pipes or hoses (not shown) are screwed. The body 11 and cover 12 are maintained in their illustrated abutting position by means such as bolts (not shown).

A cylinder assembly is clamped between a shoulder 15 in the body 11 and a shoulder 16 in the cover 12, and the cylinder assembly is made up of a cylinder barrel block 17 and a guide block 18. The blocks 17 and 18 are annular in form and define with the cover 12 an inlet chamber 19 which communicates with the inlet port 13 by way of a passage 20 formed in the cover 12. The cylinder barrel block 17 defines with the cover 12 a discharge chamber 21 which communicates with the outlet port 14 by way of a passage 22 formed in the cover 12.

The guide block 18 is restrained from any tendency to rotate by means of a retainer element 23 having an enlarged portion 24 threadably received in an internally threaded boss-like portion 25 of the body 11 and extending within a cavity 26 in the block 18.; The retainer member 23 is provided with a central bore 27 in alignment with a bore 28 inthe block member 18. The function of the bores 27 and 28 will be later described. However, it' may be seen that the internally screw-threaded portion 25 of the body 11 may receive a suitably threaded hose or pipe. V i

The cylinder barrel block 17 is bored through to form a plurality of radially arranged cylinders 29, and the block 17 is formed to define with the block 18 inlet passages 30 which communicate with the cylinders 29. While only one cylinder is illustrated in the drawings, the usual number of cylinders is nine (9). The guide block 18 is bored through, at 31, these guide bores 31 being of the same diameter and in alignment with the cylinders 29. A discharge valve 32 associated with each cylinder 29, is seated by a coiled compression spring 33 against the outer face of the cylinder barrel block 17. The discharge valves 32 are guided in pockets 34 formed in the cover 12, and are disposed within the discharge chamber 21, as illustrated. a v

A piston or plunger 35 is slidably received in, each of the cylinders 29 and extends through a guide bore 31 and the block member 18, terminating in an enlarged shoulder portion 36 located in a cavity 37 within the body 11 and behind the block 18. Each piston is provided with a cavity 38 open at the end thereof adjacent the associated discharge valve 32, a cylindrically elongated inlet port 39 in communication with the inlet passage 30 when the piston is in the position illustrated in Figure 1, and an elongated slot 40 for a purpose to be described.

A swash plate or cam 41 disposed within the cavity 37 isprovided for reciprocating the pistons or plungers 35 between the position shown and a position in which the pistons are fully displaced within their cylinders 29, which positions correspond, respectively, to full suction and full discharge positions. The cam 41 is provided with a hub 42, journalled in suitable needle bearings 43, to-which-isconnected, by axial thrust means 44, a splined driving shaft 45. The thrust means 44 being convert tional needs no description.

Each piston 35 is provided with a substantially hemis pherical cavity 46 in the enlarged portion 36 to receive a complementary-shaped bearing shoe 47 having a flat surface which bears against the cam 41. The needle bearings 43 are retained in an annular retainer member 48, fixedly positioned in the body 11, which is also pro vided with hemispherical cavities 49 to receive complementary-shaped bearing shoes 50 having flat surfaces to engage the opposite side of the cam 41 from the bearing shoes 47. The pistons 35 are retracted by means of a piston return plate 51 having fork-like portions 52 engageable with the enlarged portions 36 of the pistons. The piston return plate is provided with a centrally located hemispherical cavity 53 which is disposed over a complementary-shaped end portion 54 of a pilot valve retainer member 55, the purpose of which will be later explained, received in a central bore 56 in the guide block 18. The portion 54 of the pilot valve retainer maintains the piston return plate 51 in its operating position.

A valve member 57 having a frustoconical portion 58 adapted to seat on a correspondingly shaped surface portion 59 of the piston 35 is provided for closing the open end of each piston 35 during at least a portion of the pressure or discharge stroke of the piston. The valve member 57 is provided with an axially extending stem member 60' having an enlarged end portion 61 and is slidably supported within the piston 35 by means of a support member 62 fixedly disposed within the cavity 38 of each piston 35. A coil compression spring 63 surrounds the stem portion 60 and is disposed between the enlarged end portion 61 and the support member 62 to resiliently urge the valve member to its illustrated position, whereinthe frusto-conical portion 58 is seated on the surface 59. As will be obvious by varying the position, relative to the cylinder 29, in which the valve 57 closes the open end of a piston, as by seating on the surface 59, the effective discharge stroke of the piston 35 will be varied, and such variation will vary the displacement of the pump.

In order to vary the position at which a valve 57 seats on a surface 59 of a piston 39, means in the form of separate, individual finger-like elements, generally indicated at 64, are provided, one for each piston. Each element 64 comprises a radially extending portion 65, the outermost end of which extends through the elongated slot 40 in a piston to aposition wherein it may contact the enlarged valve stem portion 61. Each element 64 is axially movable between certain limits corresponding to full and zero pump displacement and is supported for such axial movement by a cylindrical stem 66 formed integrally with the portion and which is slidably received in a bore 67 formed in the block 18.

An annular piston 68 is disposed within a cavity 69 in the guide block 18 and defines with the cavity 69 a chamber 70 which communicates with the bore 67 by means of a passage 71, also formed in the block 18. It:should be noted :here thatthe passage 28 previously described communicates with the chamber 70, as illustrated.

The piston 68 is provided with axially extending cylindrical portions 72 and 73 of different diameters, defining at their juncture a stop shoulder 74. A cylindrical, flanged, retainer member 75 is slidably received around the cylindrical extension 73, so that the sliding movement of the retainer member is limited in one direction by the shoulder 74.

A coil compression spring 76, hereinafter referred to as a finger element return spring is received between the retainer member 75 and a closure member 77 received in an opening 78 in the cover 12. The closure member 77 is provided with a reduced cylindrical axially extending portion 79 which supports the reduced cylindrical portion 73 of the piston 68 in telescopic arrangement, as illustrated. The finger elements 64 are guided in their axial movement and refrained from turning by means of spaced fingers 80 extending from and integral with the guide block 18.

The pilot valve retainer 55, received in the bore 56, as is obvious from the drawings, also includes a portion received within the annular piston 68. The retainer member 55 is provided with open-ended concentric coinmunicating bores 81 and 82, the bore 82 being of smaller diameter than the bore 81, and is transversely drilled as at 83, to form a communication between the outer peripheries thereof and the bore 82. The retainer member 55 is also provided with a reduced annular groove portion 84 intermediate its ends and which defines with the piston 68 and the bore 56 a chamber 85, for a purpose to be described. It should be noted that the chamber 85 is in communication with the piston chamber 70. The retainer member 55 is also provided with transverse bores or openings 86 and 87 providing communication between the cavity 82 and the chamber 85.

A pilot valve 88 is slidably received within the cavity 82 and is provided with an open-ended cavity 89, a first groove 90, a second groove 91, a third groove 92, and a reduced cylindrical portion 93 defining a shoulder 94 with the groove 92. A pilot valve sleeve 95 is disposed within the cavity 82 surrounding the reduced portion 93 of the pilot valve 88 and is fixedly retained in position by means of a fastening element 96. The sliding movement of the pilot valve 88 is limited by the shoulder 94 engaging the pilot valve sleeve 95 in one direction and the end of the pilot valve 88 engaging the shoulder formed by the intersection of the bore 31 and 82 in the other direction. The pilot valve 88 is transversely bored, as at 97, to provide communication between the groove 91 and the bore 89, and the groove 92, together with a part of the reduced portion 93 and the pilot valve sleeve 95, defines with the bore 82 a pilot ring pressure chamber 98.

A pin member 99 is disposed within the cavity 89 at one end and within a cavity 100 in a cup-like spring retainer member 101 at its other end. The spring retainer member 101 is elongated and has annular portions slidably engageable with the interior of the cylindrical member 83 extending from the piston 68. A coil compression spring 102, hereinafter referred to as a pilot valve spring, is disposed within the cup-like spring retainer member 101 at one' end and at its other end is supported by means of a tiplike portion 103 extending from a plug portion 104 of an adjusting screw 105 threadably received in the member 77, as illustrated. The adjusting screw 105 extends to a position outside the cover 12 and is provided with a slot 106 to receive means, such as a screw driver, to adjust the position of the screw member 105 and thereby adjust the loading of the spring 102. The s ring 102, as is obvious, resiliently urges the pilot valve 88 to its illustrated position (see Figure 2) through the pin member 99. An acorn nut 107 is threadably received over the screw-threaded adjusting screw 105 to prevent damage thereto.

It will be noted that openings are provided in the portion 73 and in the retainer 101 for the passage of fluid therethrough, this becoming apparent from the operation of the pump, later to be described.

The guide block 18 is provided with an annular chamber 108 in communication with the openings 83, and is also provided with an annularly disposed passage 109 in communication at one end with the cavity 108 and at the other end with a passage 110 formed in the cylinder barrel block 17. The passage 110 in the cylinder barrel block 17 communicates with the discharge chamber 21 for the passage of discharge pressure to the cavity 108.

An air lock release valve, generally indicated at 111 is provided, but since this does not form a part of my invention herein described further description is unnecessary, except to say that the valve 111 is identical to that described and claimed in my aforementioned copending application, to which reference is made.

Suitable O-ring sealing means are provided where necessary as is usually the practice in devices of this nature.

The following description sets forth the operation of the pump and this operation will be described with reference to one of the plurality of pistons, it being understood that the operation of all pistons is the same.

Assuming the shaft 45 is rotating, it will be apparent that the cam or swash plate 41 will be rotating and the pistons 45 will be reciprocated Within the cylinders 34 between their suction positions and their discharge positions. Fluid from a suitable source will flow to the inlet 13, into the passage 20 and into the chamber 19 filling the entire chamber. From the chamber 19 the fluid will flow through the passage 30, and when the piston is in its suction position, through the openings 39 and 40 and into the cavity 38 of the piston 35. The fluidwithin the piston cavity 38 exerts a force on the piston valve 57 causing it to unseat and move away from the end of the piston, so that fluid will flow around the valve 57 and into the chamber 29 in advance of the piston, the fluid being retained therein by means of the seated discharge valve 32. As is obvious, on the discharge stroke of the piston 35 the fluid within the chamber 29 will be compressed causing the valve 57 to close.

As the piston moves to the left in its discharge stroke, compressing the fluid trapped within the chamber 29, the discharge valve 32 will be unseated from the end of the block 17, so that the fluid under pressure will flow into the discharge chamber 21, through the discharge passage 22 and out the outlet 14. At the same time, high pressure fluid from the discharge'chamber 21 will flow through the passage 110, through the passage 109, and into the chamber 108. The fluid in the chamber 108 will flow through the openings 83 and into the bore 81 where it contacts one end of the spool-type pilot valve 88.

When the pressure in the bore 81 becomes sufliciently high, or, in other words, the discharge pressure becomes sufliciently high, so that the force on the end of the pilot valve 88 overbalances the spring 102, the pilot valve 88 moves to the left, as illustrated in the drawings, and compresses the pilot valve spring 102. The pilot valve spring 102 may be adjusted by means of the screwthreaded, slotted member 105 to a load which will allow the pilot valve 88, to reach a position providing a bleed between the groove 90 and the openings 87 at a predetermined outlet pressure. At this point a slight increase in outlet pressure will allow the flow of fluid inv the groove 90 to bleed into the piston chamber 70 through the opening 87. At this time it will be noted that the groove 91 is blocked by the interior of the bore 82. Pressure will build up in the piston chamber 70 suflicient to overcome the return spring 76, and the fluid thus acting on the piston 68 will start moving the piston 68 to the left, as illustrated in the drawings. Fluid in the chamber 70 will also flow through the passage 71 and into the bore 67 behind 6 the stem 66 and act on the stem 66 moving the element 64 to the left.

After the element 64 has moved a short distance, its finger portion 65 contacts the enlarged end 61 of the piston valve 57 and forces the valve 61 to open for part of the pressure stroke. This allows fluid in the piston chamber 29 to bypass back to inlet and also prevents pressure in the piston chamber 29 from building up until the piston has reached a point in its travel toward its discharge position where it picks up the valve portion 58 and pulls the valve away from the finger portion 65.

A slight decrease in outlet or discharge pressure will allow the pilot valve 88 to move toward its position illustrated and thereby shut otf the outlet flow to the chamber 78 and behind the stem portion 66 of the element 64, and will allow fluid trapped in the chamber 70 and in the bore 67 to bleed out through the passage 82, into the groove 91, through the passages 97, through the bore 89 and to the chamber 19 through the aforementioned openings in the cup-like retainer member 101 and openings in the cylindrical portion 73. The return spring 76 acting through the retainer member 75 will-then start returning the elements 64 to their illustrated position, which will allow the piston valve 57 to close off sooner, thereby increasing the eifective piston stroke which in turn increases the displacement of the pump.

A steady, partial flow demand will cause the pilot valve 88 to assume a position in which pilot valve leakage or bleed into the piston chamber 70 will equal pilot valve leakage out of the piston chamber 70 back to the inlet cavity 19 through the groove 91, passages 97 and bore 89, thus holding the elements 64 in a constant partial flow position. Since the pilot valve 88 travels only a few thousandths of an inch in the bore 82 to bleed flow into and out of the chamber 70, the pilot valve spring load increase due to this travel is very slight. Also, since the pilot valve 88 assumes the same position for any partial flow demand, the load on the spring 76 is the same and therefore, the outlet pressure will be the same. Thus, there would be no pressure differential between maximum pressure at full flow and at zero flow. But, there must be a pressure change in order to control the location of the elements 64, and this pressure change, also called the pressure differential, has been built into the pilot valve 88. Thus, a cut-off slope or angle is provided on a pressure versus flow curve, which corresponds to the pressure differential.

To provide the pressure differential is the function of the pilot ring pressure chamber 98. As will be obvious, when fluid flows to the chamber 70, there will also be a flow to the chamber 85, through the opening 87 and into the chamber 98. When this chamber is under pressure, it balances out part of the outlet pressure which contacts the end of the pilot valve 88. The pressure in the chamber 98 increases as the elements 64 move towards zero displacement position (the limit of the movement of the elements 64 to the left, as illustrated) due to the increasing return spring load. As the pressure increases this creates the pressure diflerential between full and zero flow. I

Thus, the pilot ring pressure chamber 98 hydraulically applies the spring rate of the return spring 76 to the pilot valve 88. During partial displacements of the pump, the return spring load varies with the position of the elements 64. As the return spring load varies, the pressure in the piston chamber 70 and in the pilot ring chamber 98 varies. Since the pilot ring chamber 98 applies this pressure to the pilot valve 88 against outlet or discharge pressure, the outlet pressure must vary to maintain the pilot valve 88 in balance. Therefore, a definite link has been established between the location of the elements 64 and outlet pressure and since the location of the elements 64 establishes the correct dis- 7 placement, it can be said that outlet pressure establishes the correct displacement.

Without this pilot ring chamber 98, the valve 88 would crack at a set pressure and close off at a set pressure regardless of the location of the elements 64. This is true because the elements 64 can move from the full displacement position (as illustrated) to the zero displacement position (the limit of their movement to the left) without increasing the load of the pilot valve spring 192. Therefore, there would be no change in outlet pressure whether the pump be in, for example, 75% displacement, 50% displacement or zero displacement. if there were no change in outlet pressure, there would be no means of stopping the elements 64 at any desired location, and as a result, the elements 64 would hunt back and forth, resulting in undesirable chatter.

It is also possible in the instant pump to provide for zero flow and zero outlet pressure by connecting a low pressure source of fluid to the passages 27 and 28, so that the low pressure fluid will act upon the piston 63 and move the elements 64 to their zero flow position, at which position the inlet valves will be held open for the full amount of the pressure stroke.

If when an element 64 picks up a piston valve stem, the pressure in the chamber 29 is very high, the element 64 will not move the valve 57, because of the relatively low force acting on the stem 66, thereby avoiding the breaking of the portion 65 therefrom.

While I have described my invention in connection with certain specific embodiments thereof, it is to be understood that this is by way of illustration and not by way of limitation and the scope of my invention is defined solely by the appended claims which should be construed as broadly as the prior art will permit.

I claim:

l. A hydraulic apparatus comprising: a housing having a cavity therein; a block disposed in said cavity and havmg a plurality of cylinders formed therein; means defining an inlet and an outlet leading respectively to and from said cylinders; a plurality of pistons disposed respectively in said cylinders; means defining an openended cavity in each piston; a valve carried by each piston and adapted to seat on the open end thereof to close said open end, each valve including a stem portion and being resiliently urged to its seated position; means to reciprocate said pistons to provide intake and discharge strokes for each piston whereby each piston is adapted to displace fluid from the inlet to the outlet by drawing fluid into the respective cylinder during its intake stroke, trapping the fluid in the cylinder when the respective valve is seated, and forcing the trapped fluid out through the outlet during the discharge stroke thereof; a plurality of movable members disposed in said housing and being adapted upon movement thereof to contact respectively said stem portions to thereby unseat said valves during at least a portion of said discharge strokes of said pistons and thus vary the amount of fluid trapped thereby; means providing a plurality of chambers disposed in said housing, said chambers being respectively defined in part by said members; and means to communicate pressure fluid to said chambers to cause movement of said members relative to said housing and thus vary the displacement of said apparatus.

2. A hydraulic apparatus comprising: a housing having a cavity therein; a block disposed in said cavity and having a plurality of cylinders formed therein; means defining an inlet and an outlet leading respectively to and from said cylinders; a plurality of pistons disposed respectively in said cylinders; means defining an open-end d cavity in each piston; a valve carried by each piston and adapted to seat on the open end thereof to close said open end, each valve including a stem portion and being resiliently urged to its seated position; means to reciprocate said pistons to provide intake and discharge strokes for each piston whereby each piston is adapted to displace fluid from the inlet to the outlet by drawing fluid into the respective cylinder during its intake stroke, trapping the fluid in the cylinder when the respective valve is seated, and forcing the trapped fluid out through the outlet during the discharge stroke thereof; a plurality of movable members disposed in said housing and being adapted upon movement thereof to contact respectively said stem portions to thereby unseat said valves duringat least a portion of said discharge strokes of said pistons and thus vary the amount of fluid trapped thereby, said members being movable between positions of maximum and minimum displacement;

eans providing a plurality of chambers disposed in said housing, said chambers being respectively defined in part by said members; means to apply and vent pressure fluid respectively to and from said chambers whereby the force of the pressure fluid conveyed to said chambers causes movement of said members toward their minimum positions to thus decrease the displacement of said apparatus; and means adapted to engage said members and move said members toward their maximum positions when said pressure fluid in said chambers is vented.

3. A hydraulic apparatus comprising: a housing having a cavity therein; a block disposed in said cavity and having a plurality of cylinders formed therein; means defining an inlet and an outlet leading respectively to and from said cylinders; a plurality of pistons disposed respectively in said cylinders; means defining an open-ended cavity in each piston; a valve carried by each piston and adapted to seat on the open end thereof to close said open end, each valve including a stem portion and being resiliently urged to its seated position; means to reciprocate said pistons to provide intake and discharge strokes for each piston whereby each piston is adapted to displace fluid from the inlet to the outlet by drawing fluid into the respective cylinder during its intake stroke, trapping the fluid in the cylinder when the respective valve is seated, and forcing the trapped fluid out through the outlet during the discharge stroke thereof; a plurality of movable members disposed in said housing and being adapted upon movement thereof to contact respectively said stern portions to thereby unseat said valves during at least a portion of said discharge strokes of said pistons and thus vary the amount of fluid trapped thereby, said members being movable between positions of maximum and minimum displacement; means providing a plurality of chambers disposed in said housing, said chambers being respectively defined in part by said members; means to apply and vent pressure fluid respectively to and from said chambers whereby the force of the pressure fluid conveyed to said chambers causes movement of said members toward their minimum positions to thus decrease the displacement of said apparatus; and movable spring means carried by said housing adapted to engage said members to limit movement thereof toward their minimum positions and to return said members toward their maximum positions when said pressure fluid in said chambers is vented.

4. A hydraulic apparatus comprising: a housing having a cavity therein; a block disposed in said cavity and having a plurality of cylinders formed therein; means defining an inlet and an outlet leading respectively to and from said cylinders; a plurality of pistons disposed respectively in said cylinders; means defining an open-ended cavity in each piston; a valve carried by each piston and adapted to seat on the open end thereof to close said open end, each valve including a stern portion and being resiliently urged to its seated position; means to reciprocate said pistons to provide intake and discharge strokes for each piston whereby each piston is adapted to displace fluid from the inlet to the outlet by drawing fluid into the respective cylinder during its intake stroke, trapping the fluid in the cylinder when the respective valve is seated, and forcing the trapped fluid out through the outlet during the discharge stroke thereof; a plurality of movable members disposed in said housing and being adapted upon movement thereof to contact respectively said stem por' tions to thereby unseat said valves during at least a portion of said discharge strokes of said pistons and thus vary the amount of fluid trapped thereby, said members being movable between positions of maximum and minimum displacement; means providing a plurality of chambers disposed in said housing, said chambers being respectively defined in part by said members; means to apply and vent pressure fluid respectively to and from said chambers whereby the force of the pressure fluid conveyedto said chambers causes movement of said members toward their minimum positions to thus decrease the displacement of said apparatus; and movable spring means carried by said housing adapted to engage said members to limit movement thereof toward their minimum positions and to return said members toward their maximum positions when said pressure fluid in said chambers is vented, said spring means including a piston having a surface subject to the application of the force of the pressure fluid conveyed to said chambers whereby the force of said pressure fluid causes said piston to move said spring means in opposition to the force of said spring means between positions corresponding to said maximum and minimum positions of said members and thus determines the displacement of said apparatus. 7

5. A hydraulic apparatus comprising: a housing having a cavity therein; a block disposed in said cavity and having a plurality of cylinders formed therein; means defining an inlet and an outlet leading respectively to and from said cylinders; a plurality of pistons disposed respectively in said cylinders; means defining an open ended cavity in each piston; a valve carried by each piston and adapted to seat on the open end thereof to close said open end, each valve including a stem portion and being resiliently urged to its seated position; means to reciprocate said pistons to provide intake and discharge strokes for each piston whereby each piston is adapted to displace fluid from the inlet to the outlet by drawing fluid into the respective cylinder during its intake stroke, trapping the fluid in the cylinder when the respective valve is seated, and forcing the trapped fluid out through the outlet during the discharge stroke there- 10 of; a plurality of movable members disposed in said housing and being adapted upon movement thereof to contact respectively said stem portions to thereby unseat said valves during at least a portion of said discharge strokes of said pistons and thus vary the amount of fluid trapped thereby, said members being movable between positions of maximum and minimum displacement; means providing a plurality of chambers disposed in said housing, said chambers being respectively defined in part by said members; means to apply and vent pressure fluid respectively to and from said chambers whereby the force of the pressure fluid conveyed to said chambers causes movement of said members toward their minimum positions to thus decrease the displacement of said apparatus; movable spring means carried by said housing adapted to engage said members to limit movement thereof toward their minimum positions and to return said members toward their maximum positions when said pressure fluid in said chambers is vented, said spring means including a piston having a surface subject to the application of the force of the pressure fluid conveyed to said chambers whereby the force of said pressure fluid causes said piston to move said spring means in opposition to the force of said spring means between positions corresponding to said maximum and minimum positions of said members and thus determines the displacement of said apparatus; and adjustable means to vary the force of said spring means.

References Cited in the file of this patent UNITED STATES PATENTS 2,385,784 Baker Oct. 2, 1943 2,405,006 Ashton July 30, 1946 2,444,550 Ashton July 6, 1948 2,459,303 Baker Jan. 18, 1949 2,512,799 Huber June 27, 1950 2,562,615 Huber July 31, 1951 2,576,263 Pielstick Nov. 27, 1951 2,664,047 Huber Dec. 29, 1953 2,732,805 Lucien Jan. 31, 1956 

